This invention relates to a beam track position control method and apparatus for an optical disk apparatus, and more particularly to a beam track position control method and apparatus for an optical disk apparatus wherein two beams including a write beam and a read beam pass through a single object lens of an optical head such that, while data are being written onto a certain track of an optical disk with the write beam, the information written with the write beam is read out with the read beam, which is positioned rearwardly of (following) the write beam in the direction of rotation of the disk.
A verify read operation is carried out with a conventional optical disk apparatus. Such a verify read involves reading, after data have been written onto an optical disk, of such data to compare the read out data with the write data to assure the reliability of the optical disk apparatus.
In conventional systems, a single beam of light is projected onto an optical disk by way of an object lens of an optical head so that it may serve for both of writing and reading. More particularly, using a single beam of light, writing is carried out for a certain one rotation of an optical disk, and then, reading is carried out for a next one rotation of the optical disk, whereafter the write data and the read data are compared with each other. With an optical disk apparatus which carries out such verify read, an optical disk must be rotated by two complete rotations for the writing of data, and there is a problem that much time is required.
In recent years, a technique has been proposed wherein a write beam and a read beam pass through a single object lens of an optical head at the same time such that, while data are being written onto a certain track of an optical disk with the write beam, the information written with the write beam is read out by the read beam which is positioned rearwardly of the write beam in the direction of rotation of the disk. Consequently, writing of data and verify read are carried out at the same time to thereby reduce the data writing time of the optical disk apparatus. In the optical disk apparatus, focusing of a beam of light is controlled by a focusing servo controller while the track position of the beam is controlled by a track servo controller simultaneously for both the write beam and the read beam. Track servo control makes use, for example, of a variation of the amount of reflected light due to a diffraction phenomenon of a beam spot by a spiral guide groove (track) provided in advance on an optical disk. In particular, making use of the fact that a reflected light amount distribution detected by a photodetector is varied with a diffraction of light by a track depending upon a position of a beam spot relative to the track, a track error signal of a beam spot with respect to the track is obtained.
Such track error signal is obtained by receiving an amount of reflected light of a write beam by means of a photodetector. Conventionally, in an optical disk apparatus wherein two beams of light are projected onto an optical disk through a single object lens to carry out writing and reading of data at the same time, a track error signal is obtained only from the write beam to control the track positions of the two beams at the same time.
With this controlling method, however, it has been discovered that, while control in the focusing direction is sufficient for practical use, a read beam is sometimes displaced in a direction across tracks so that the beam reads, for example, an adjacent track. It can be considered that this displacement of a read beam is caused by a variation of the index of refraction, time aging, etc. of a lens or the like due to a temperature variation in an optical head.
Once such displacement of a read beam with respect to a track takes place, even if a write beam is projected onto a predetermined track position, the read beam will not be positioned on the same track as the write beam. Consequently, it is impossible to carry out verify read with a read beam during data writing with the write beam.